Issue 565: MVStore: concurrently adding LOB objects (with MULTI_THREADED option) resulted in a NullPointerException. 

There was a way to prevent a database from being re-opened, by creating a column constraint that references a table with a higher id.

If a database file in the PageStore file format exists, this file and this mode is now used, even if the database URL does not contain "MV_STORE=FALSE". If a MVStore file exists, it is used.

Databases created with version 1.3.175 and earlier that contained foreign keys in combination with multi-column indexes could not be opened in some cases.